This invention relates generally to a system and method for producing metallic iron by thermally reducing an iron oxide with a carbon bearing reductant in a moving hearth furnace.
Many different iron ore reduction processes and furnaces have been described and/or used in the past. These processes may be traditionally classified into direct reduction processes and smelting reduction processes. Generally, direct reduction processes convert iron ores into a metallic form with, for example, use of shaft furnaces (e.g., natural gas-based shaft furnaces), whereas smelting reduction converts iron ores into molten hot metal without the use of blast furnaces. The shaft furnace processes include the Midrex® process where an iron oxide source is reduced in a furnace by blowing a reducing gas, e.g., a natural gas, through a tuyere disposed at a lower portion of the shaft furnace. A SL/RN process is another example of a direct iron making process. In the SL/RN process, a carbon bearing material such as coal is used as the reducing agent, and the carbon material is heated together with the iron oxide source, e.g., iron ores, in a rotary kiln to reduce the iron oxide source.
The conventional reduction processes for production of direct reduced iron (DRI) involve heating beneficiated iron ores to below the melting point of iron, below 1200° C. (2372° F.), either by gas-based processes or coal-based processes. For example, in the gas-based process, direct reduction of iron oxide (e.g., iron ores or iron oxide pellets) employs the use of a reducing gas (e.g., reformed natural gas) to reduce the iron oxide and obtain DRI. Methods of making DRI have employed the use of materials that include carbon such as coal and coke as a reducing agent. A typical composition of DRI is 90 to 95% metallization and 2-4% gangue, but has not been practical for use in steelmaking processes as a replacement of scrap because its oxygen and gangue content increases energy usage, increase slag volume, and necessitates the addition of costly reagents.
Natural gas-based direct reduced iron accounts for over 90% of the world's production of DRI. Coal-based processes are generally used in producing the remaining DRI production. However, in many geographical regions, the use of coal may be more desirable because coal prices may be more stable than natural gas prices. Further, many geographical regions are far away from steel mills that use the processed product.
Another gas-based or coal-based reduction process for directly reducing iron bearing material to metallic nodules is often referred to as fusion reduction. Such fusion reduction processes, for example, generally involve the following processing steps: feed preparation, drying, preheating, reduction, fusion/melting, cooling, product discharge, and metallic iron/slag product separation. These processes result in direct reduction of iron bearing material to metallic iron nodules and slag. Metallic iron nodules produced by these direct reduction processes are characterized by high grade reduction, nearing 100% metal (e.g., about 96% to about 97% metallic Fe).1 1Percents (%) herein are percents by weight unless otherwise stated.
Unlike conventional direct reduced iron (DRI), these metallic iron nodules have low oxygen content because they are metallic iron and have little or no porosity. These metallic iron nodules are also low in gangue because silicon dioxide has been removed as slag. Such metallic iron nodules are desirable in many circumstances such as use in place of scrap in electric arc furnaces. These metallic iron nodules can be also produced from beneficiated taconite iron ore, which may contain 30% oxygen and 5% gangue. As a result, with such metallic iron nodules, there is less volume to transport than with beneficiated taconite pellets or DRI. In addition, generally, such metallic iron nodules are just as easy to handle as taconite pellets and DRI.
Various types of hearth furnaces have been described and used for direct reduction of metallic iron nodules. One type of hearth furnace, referred to as a rotary hearth furnace (RHF), has been used as a furnace for coal-based direct reduction. An example of such a rotary hearth furnace is described in U.S. Pat. No. 3,443,931. Another type is the linear hearth furnace such as described in US 2005/229748.
Both the rotary hearth furnace and the linear hearth furnace involve making mixtures of carbon bearing material with iron ore or other iron oxide fines into balls, briquettes or other compacts, and heating them on a moving hearth furnace to reduce the iron oxide to metallic iron nodules and slag. Typically, both the rotary and linear hearth furnaces are partitioned into a preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the supply location and the discharge location of the furnace. In operation, raw reducible material comprising a mixture of iron ore and reducing material is charged onto the moving hearth and moved into the preheat zone where the raw materials are dried and preheated. After preheating, the iron ore mixture on the hearth is moved to the reduction zone where the iron ore is reduced in the presence of the reducing material and fused into metallic iron nodules, using one or more heat sources (e.g., gas burners). The reduced and fused product, after completion of the reduction process, is cooled in the cooling zone on the moving hearth, preventing oxidation and facilitating discharge from the furnace.
A limitation of these furnaces, and the methods of operating these furnaces, in the past has been their energy efficiency. The iron oxide bearing material and associated carbon bearing material generally had to be heated in the furnace to about 2500° F. (1370° C.), or higher, to reduce the iron oxide and produce metallic iron material. The furnace generally required natural gas or coal to be burned to produce the heat necessary to heat the iron oxide bearing material and associated carbon bearing material to the high temperatures to reduce the iron oxide and produce a metallic iron material. Furthermore, the reduction process involved production of volatiles in the furnace that had to remove from the furnace and secondarily combusted to avoid an environmental hazard, which added to the energy needs to perform the iron reduction. See, e.g., U.S. Pat. No. 6,390,810.
What has been needed is a furnace that reduces the energy consumption needed to reduce the iron oxide bearing material such that a large part, if not all, of the energy to heat the iron oxide bearing source to the temperature necessary to cause the iron oxide to be reduced to metallic iron and slag comes from combusting volatiles directly in the furnace itself, and otherwise using heat generated in one part of the furnace in another part of the furnace. Such a furnace is described in Provisional Application Ser. No. 60/828,170, filed Oct. 4, 2006, which is incorporated herein by reference. Still there is a need for a furnace that has a higher production capacity, is more efficient in transferring fluids between different parts of the furnace, and has a lower capital and operating cost for a given production capacity.